Lubricant additives, especially automotive additives such as viscosity index improvers and detergent-inhibitor (DI) packages require lubricant additive diluent oils to make them useable. Accordingly, lubricant additive diluent oils are used to dissolve lubricant additives to provide oil soluble additive concentrates. These oil soluble additive concentrates make the additives easier to transport, handle, and ultimately blend into lubricant base oils to provide a finished lubricant. The oil soluble additive concentrates are not useable or suitable as finished lubricants on their own. Rather, the oil soluble additive concentrates are blended with lubricant base oil stocks to provide a finished lubricant. It is desired that the lubricant additive diluent oils readily solubilize the lubricant additive and provide an oil additive concentrate that is readily soluble in the lubricant base oil stocks. In addition, it is desired that the lubricant additive diluent oils not introduce any undesirable characteristics, including, for example, high volatility, high viscosity, and impurities such as heteroatoms, to the lubricant base oil stocks and thus, ultimately to the finished lubricant.
Different lubricant additive diluent oils require differing amounts of lubricant additive diluent oil to provide a suitable oil soluble additive concentrate. By way of example, oil soluble additive concentrates comprising gear oil additive packages may contain as little as 25% weight lubricant additive diluent oil. Oil soluble additive concentrates comprising DI packages typically contain about 50% weight lubricant additive diluent oil. Oil soluble additive concentrates comprising viscosity index improver typically contain about 90% weight or more lubricant additive diluent oil.
Currently, the lubricant additive diluent oils used with most DI packages and viscosity index improvers are highly aromatic base oils that fall into API Group I. API Group I base oils, with their high solvency and good availability, have been preferred as lubricant additive diluent oils. However, these Group I oils have only average to poor low temperature performance, and they are much more susceptible to oxidation than modern oils, which are more highly saturated. In addition, Group I base oils have lower viscosity indexes (VI) and higher volatility than other base oils. Moreover, Group I base oils have high sulfur concentrations. Lubricant additive diluent oils can comprise up to 5 to 10 weight percent of a finished lubricant. Accordingly, the properties of the lubricant additive diluent oils are important as undesirable properties in the lubricant additive diluent oils can negatively impact the properties of the finished lubricant. Although more desirable in terms of their properties for the finished lubricant, conventional API Group II, conventional Group III, and Group IV base oils are difficult to use as lubricant additive diluent oils due to their poor ability to solubilize additives. Therefore, these base oils are not practical as lubricant additive diluent oils.
When added to lubricant base oil stocks, typical oil soluble additive, concentrates comprising DI packages or viscosity index improvers, tend to thicken the finished lubricant formulation and impair its low-temperature performance. Low viscosity lubricant additive diluent oils, which have been used in the past in an attempt to avoid thickening the finished lubricant, have either had high volatility or poor additive solubility, making them unsuitable for most applications. When added to engine oils, the typical oil soluble additive concentrates tend to adversely impact the cold-cranking simulator (CCS) viscosity and Mini-Rotary Viscometer (MRV). When added to automatic transmission fluid and gear oils, the typical oil soluble additive concentrates tend to adversely impact the Brookfield Viscosity at low temperature.
Accordingly, lubricant additive diluent oils with low viscosity, low volatility, and low concentrations of impurities, such as sulfur-containing compounds, are desired. Typical lubricant base oils with low volatilities also have high viscosities rendering them unsuitable for most applications, and typical lubricant base oils with low viscosities also have low volatilities and poor additive solubility rendering them unsuitable for most applications.
Engine manufacturers worldwide are introducing chemical limits on engine oils and additives that they believe will provide the safe margins for operation that their exhaust aftertreatment hardware requires. These requirements will directly impact what is suitable for use as additives, lubricant base oils stocks, and lubricant additive diluent oils. Low sulfur and phosphorus limits on engine oils are being proposed. At a limit of about 0.3 weight % sulfur, zinc dithio-diphosphate antiwear additives need to be partially replaced with more costly additives, and reduced sulfur detergents and base oils are needed to provide formulation flexibility. As limits move toward 0.2 weight % sulfur, reduced or zero-sulfur lubricant base oils and diluent oils become essential to meet formulation targets. International Lubricants Standardization and Approval Committee (ILSAC) GF-4 passenger car engine oils, API PC-10 heavy duty engine oil, and other high quality finished lubricant specifications call for low sulfur formulations.
The ILSAC/Oil Committee adopted the new GF-4 specification for passenger car motor oils on Jan. 8, 2004, with a recommended start date for introducing GF-4 into the marketplace of Jul. 1, 2004. A new bench test requirement for engine oils meeting the GF-4 specification are maximum sulfur content by ASTM D 1552. As such, a 10 W oil may have a maximum of 0.7 weight % sulfur, while 0 W, and 5 W oils may have a maximum of 0.5 weight % sulfur. In addition, the oils meeting the GF-4 specification must have a Noack volatility by ASTM D 5800 of less than 15 weight % after one hour at 250° C., and a simulated distillation by ASTM D 6417 with a maximum of 10% at 371° C. API PC-10 is a proposed specification for heavy duty diesel engine oil and is expected to be approved in 2006 or 2007. It is expected that PC-10 oils will also have reduced limits for sulfur, similar to those amounts called for GF-4 passenger car motor oils.
Accordingly, lubricant additive diluent oils with low sulfur, excellent additive solubility, good elastomer compatibility, low volatility, low viscosity, high oxidation stability, good low temperature properties, and excellent solubility in the lubricant base oils are desired.